Pursuant to 35 U.S.C. xc2xa7119(a)-(d), this application claims priority from Taiwanese application no. 91100577, filed on Jan. 16, 2002.
1. Field of the Invention
The present invention relates to a driving circuit for transmission lines, particularly to a driving circuit for transmission lines which is composed of discrete components so that the cost of the driving circuit is reduced.
2. Description of the Prior Art
FIG. 1 is a diagram showing a conventional driving circuit for an RS-232 transmission line. An internal circuit 11 sends an outgoing signal Txc3x97 to a signal driver chip 12. The chip 12 transforms the outgoing signal Txc3x97 to a signal Txc3x97d large enough to be sent to an external circuit 13 by the RS-232 transmission line 14. The external circuit 13 sends an incoming signal Rxc3x97d through the RS-232 transmission line 14 to the chip 12. The chip 12 transforms the incoming signal Rxc3x97d to a signal Rxc3x97 sent to the internal circuit 11.
However, the signal driver chip 12 is very expensisve so that the conventional driving circuit of RS-232 transmission line suffers a high cost.
Therefore, the object of the present invention is to provide a driving circuit for transmission lines which is composed of cheap discrete components. This effectively reduces the cost of the driving circuit.
The present invention provides a driving circuit for transmission lines, receiving a first outgoing signal from an internal circuit and an incoming signal from an external circuit, and sending a second outgoing signal to the external circuit. The driving circuit comprises a first transistor having an emitter connected to receive a first voltage, a collector and a base, a second transistor having a collector connected to the collector of the first transistor, an emitter and a base, a third transistor having a collector connected to receive a second voltage, an emitter through which the second outgoing signal is sent and a base, a first, second, third, fourth, fifth and sixth impedance connected between a node through which the first outgoing signal is received and the base of the first transistor, the emitter and the base of the first transistor, the collector and the base of the third transistor, the base of the third transistor and the emitter of the second transistor, the emitters of the second and third transistor, and the base of the second transistor and the collector of the third transistor, a first, second and third capacitor connected between the node and the base of the second transistor, the collector of the first transistor and the emitter of the third transistor, and the emitter of the second transistor and the collector of the third transistor, and a diode having an anode connected to the emitter of the second transistor and a cathode connected to receive the incoming signal.
Thus, the expensive signal driver chip in the conventional driving circuit is replaced by cheap discrete components in the invention, which reduces the cost.